Process for fabricating hollow electroactive devices

ABSTRACT

A process for fabricating a ceramic electroactive transducer of a predetermined shape is disclosed. The process comprises the steps of providing a suitably shaped core having an outer surface, attaching a first conductor to the outer surface of the core, coating an inner conductive electrode on the the outer surface of the core such that the inner conductive electrode is in electrical communication with the first conductor, coating a ceramic layer onto the inner electrode, thereafter sintering the ceramic layer, coating an outer electrode onto the sintered ceramic layer to produce an outer electrode that is not in electrical communication with the first conductor, and then poling the sintered ceramic layer across the inner electrode and the outer electrode to produce the ceramic electrode.

RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.10/683,019, which is a division of U.S. patent application Ser. No.09/836,441, now U.S. Pat. No. 6,654,993, which claims priority fromProvisional Patent Application Ser. No. 60/221,859, filed Jul. 28, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for fabricating highlyuniform ceramic electroactive transducers. More particularly, thepresent invention relates to a process for fabricating hollow ceramicelectroactive transducers, which are essentially spherical in shape.

2. Description of the Prior Art

A large number of medical procedures which require catheters areperformed in the United States each year. Catheters are, typically,plastic tubes having a diameter of few millimeters. The uses of suchcatheters include, intravenous drug delivery, therapeutic devicedelivery, and other types of invasive procedures, such as, guidingballoon angioplasties of the leg, guiding catheters inside the heart toablate incorrectly functioning cardiac tissue, and guiding catheterswithin the uterus to inject fluid into the fallopian tubes to test fortubal blockage.

Catheters must be guided to the proper location in the subject. Onemethod of guiding catheters is to employ ultrasonic imaging. In doingso, an ultrasonic transducer is mounted on the catheter and a receiveris placed on the subject. When an ultrasound signal from the cathetermounted transducer strikes the external receiver, the signal isconverted into an electronic marker corresponding to the preciselocation of the catheter in the ultrasound image of the subject.

At present, catheters are guided with real time X-ray imaging known asfluoroscopy. The use of ultrasound imaging has not found wide acceptancedespite the advantages ultrasound has over X-ray technology.

A major encumbrance to using ultrasound imaging in guiding catheters isorientation-dependent ultrasound visibility of catheters known in theart. The orientation-dependent visibility, which plagues thistechnology, results from the wavelength of the ultrasound being severaltimes smaller than the catheter is wide. The result is a catheter thatbecomes an ultrasound reflector, in addition to being an ultrasoundtransducer.

This produces an ultrasound visibility that is highly dependent on theorientation of the mounted transducer. X-ray fluoroscopy is not hinderedby this phenomenon.

In one approach to eliminate orientation dependant ultrasonicvisibility, an omnidirectional ultrasonic transducer is directly mountedon one end of the catheter. This approach solves the problem of signalorientation dependence on catheter orientation. To be suitable for thisuse, the ultrasonic transducer must be omnidirectional, possess lowsignal loss, have a high sensitivity, and preferably be uniform inconstruction and inexpensive to produce. Use of such an omnidirectionalultrasonic transducer mounted on a catheter would thus allow forreplacement of the hazardous X-ray imaging commonly used in guidingcatheters. The end result would be a more cost efficient, less dangerousprocedure benefiting doctors, hospitals, and their patients.

Many different types of ultrasonic transducers such as thickness mode,polymer based, solid core and hollow core transducers are known in theart.

Thickness-mode ultrasonic transducers possess low signal loss, highsensitivity, and are inexpensive to produce. However, they are highlydirectional and as such, do not meet the omnidirectional requirement.

Polymer based piezoelectric materials can be easily fabricated intodifferent shapes, including an omnidirectional geometry such as asphere. However, polymer based piezoelectric materials possess highsignal losses and low electromechanical coupling coefficients, whichrender polymer based transducers unsuitable for this use.

Solid core ceramic spherical transducers are omnidirectional. However,low sensitivity makes them less than suitable.

In contrast to the above transducers, hollow sphere ceramicelectroactive transducers have the required omnidirectionality, lowsignal loss and high sensitivity required. In addition, they can beeasily matched to electronic systems. Thus, hollow sphere ceramicelectroactive transducers promise a natural solution to the problemsassociated with guiding catheters using ultrasound-imaging technology.However, given the current state of the art, it is extremely difficultto fabricate uniform hollow sphere ceramic electroactive transducers.

Presently, hollow sphere ceramic electroactive transducers can beproduced by machining and grinding bulk ceramic into hemispheres. Anelectrode is then formed on the inner surface of the hemispheres,followed by bonding of two such hemispheres together. This approach islabor intensive, high cost, and low in productivity.

U.S. Pat. No. 4,917,857 to Jaeckel et al. is directed to a process forthe production of metallic or ceramic hollow spheres to make areticulate structure. In this process, foamed polymer cores are coatedwith a suspension containing metal or ceramic particles in a bedreactor. The polymer core is later pyrolyzed to obtain a metal orceramic hollow sphere. A hollow sphere ceramic electroactive transducercan be prepared using hollow ceramic spheres obtained from this processby first opening a small hole in the sintered sphere, normally bypolishing. Next, an inner electrode is deposited on the inner surface ofthe hollow sphere through the hole. An outer electrode is then depositedon the outer surface of the sphere. The ceramic is then poled under anelectric field. However, this method, and the transducers produced bythis method have low green density. In addition, the ceramic shellscontain many pores due to the forming process that hinder subsequentsintering of the green ceramic body. The pores become defects in thesurface of the sintered sphere, leading to mechanical fractures anddecreased sensitivity when the spheres are processed into hollow sphereceramic electroactive transducers.

Additionally, the hole created is mechanically weak. This weaknesslimits the hydrostatic pressure tolerance of the transducers produced inthis fashion. When used in underwater applications, the depth capabilityof these transducers is drastically reduced.

Thus, as discussed above, the crude fabrication techniques present inthe art do not allow for hollow ceramic electroactive transducers to beproduced uniformly, in commercially significant quantities, at a costlow enough to make this technology appealing.

SUMMARY OF THE INVENTION

The present invention includes a process for fabricating a ceramicelectroactive transducer of a predetermined shape. The process includesthe steps of: (a) providing a suitably shaped core having an outersurface; (b) attaching a first conductor to the outer surface of thecore; (c) coating an inner conductive electrode on the outer surface ofthe core such that the inner conductive electrode is in electricalcommunication with the first conductor; (d) coating a ceramic layer ontothe inner electrode; thereafter (e) sintering the ceramic layer; (f)coating an outer electrode onto the sintered ceramic layer to produce anouter electrode that is not in electrical communication with the firstconductor; and (g) poling the sintered ceramic layer across the innerelectrode and the outer electrode to produce a ceramic electroactivetransducer.

The present invention further includes a process for fabricating aceramic electroactive transducer of a predetermined shape which ismulti-layered. The process includes the steps of: (a) providing asuitably shaped core having an outer surface; (b) attaching a firstconductor to the outer surface of the core; (c) coating a first innerconductive electrode on the outer surface of the core such that thefirst inner conductive electrode is in electrical communication with thefirst conductor; (d) coating a first ceramic layer having an outersurface on the first inner electrode; thereafter (e) attaching a secondconductor to the outer surface of the first ceramic layer; (f) coating asecond inner conductive electrode on the outer surface of the firstceramic layer such that the second inner conductive electrode is inelectrical communication with the second conductor without being inelectrical communication with the first conductor; (g) coating a secondceramic layer having an outer surface on the second inner electrode;thereafter (h) sintering the first and the second ceramic layers; (i)coating an outer electrode on the second sintered ceramic layer toproduce an outer electrode that is not in electrical communication withthe first conductor or with the second conductor; and (j) poling thefirst and the second sintered ceramic layers across the first innerelectrode, the second inner electrode, and the outer electrode.

The present invention still further includes a process for fabricating aplurality of ceramic electroactive transducers, which have apredetermined spatial relationship of a predetermined shape. Thisprocess includes the steps of: (a) providing a plurality of suitablyshaped cores, each having an outer surface, arranged in thepredetermined spatial relationship; (b) attaching one or more firstconductors to the cores; (c) coating an inner conductive electrode oneach of the outer surfaces of the cores such that the inner conductiveelectrodes are in electrical communication with at least one of thefirst conductors; (d) coating a ceramic layer on each of the innerelectrodes; thereafter (e) sintering the ceramic layers; (f) coating oneor more outer electrodes on each of the sintered ceramic layers toproduce a plurality of outer electrodes not in electrical communicationwith the first conductor; and (g) poling the sintered ceramic layersacross the inner electrodes and the plurality of the outer electrodes.

The present invention provides for hollow ceramic electroactivetransducers by using a core formed from a thermally decomposablematerial, and further including the step of heating the layered ceramicunder conditions sufficient to remove the core prior to the sinteringstep.

In addition, the process further provides for encapsulating the ceramicelectroactive transducer in a non-conductive material after coating withthe outer electrode.

The present invention provides a simple, low cost process to produceceramic electroactive transducers that have a uniform inner electrode.The present invention also can produce large quantities of, for example,spherical ceramic electroactive transducers having a diameter rangingfrom about several tenths of a millimeter to several meters. Inaddition, the present invention eliminates the need for machining andpolishing, is simple and economical, and substantially improves theuniformity and reproducibility of the end product.

BREIF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross sectional view illustrating the core with the firstconductor attached via a binder.

FIG. 1 b is a cross sectional view illustrating the firstconductor-attached core coated with the inner electrode.

FIG. 1 c is a cross sectional view illustrating the firstconductor-attached core, coated with the ceramic layer on top of theinner electrode.

FIG. 1 d is a cross sectional view illustrating the sintered ceramiclayer with the inner electrode attached to the first conductor, with theinner core having been thermally removed.

FIG. 2 is a cross sectional view illustrating a hollow sphere ceramicelectroactive transducer coated with an outer electrode.

FIG. 3 is a graphical representation of the measured capacitance of aradially poled hollow sphere ceramic electroactive transducer fabricatedaccording to the present invention.

FIG. 4 is a graphical representation of the measured admittance of thehollow sphere ceramic electroactive transducer of FIG. 3.

FIG. 5 is a cross sectional view illustrating the fabrication of ahollow sphere ceramic electroactive transducer with multiple conductors,ceramic layers and electrodes.

FIG. 6 a is a cross sectional view illustrating a plurality of cores inspatial relationship attached to a first conductor.

FIG. 6 b is a cross sectional view illustrating a plurality of hollowceramic electroactive transducers in spatial relationship.

DETAIL DESCRIPTION OF THE INVENTION

Referring to the figures and particularly to FIG. 1, core 1 ispreferably a thermally decomposable polymer, most preferably core 1 ismade of polystyrene. To achieve a ceramic layer with high green density,core 1, or hollow core 1 with a porous outer surface is preferred.

First conductor 2, is a metal, a metal alloy, a conducting ceramic, asemi-conductor, or a combination thereof. The first conductor ispreferably a thin metal or metal alloy wire, most preferably a Pt or Niwire, having a diameter of less than about 5 mm, preferably less thanabout 1 mm. This first conductor 2 is used to apply voltage to the innerelectrode 4.

The first conductor is attached to the core 1 and secured in positionusing a suitable binder 3. The binder 3, is preferably a thermallydecomposable binder (e.g. PVA).

The inner electrode 4 is then coated onto the outer surface of the core1. The inner electrode is either an electrical conductor or asemi-conductor, preferably a metal, and most preferably Pt, Ni, or acombination thereof. The core 1 is coated with the inner electrode,preferably by contacting the core 1 with a slurry containing a dispersedpowder of the electrode material in a solvent (i.e., a metal ink). Thepowder is preferably a metal powder having an average particle size ofabout 10 microns or smaller. The inner electrode slurry can also containat least one suitable co-solvent, binder, dispersant, antifoam, or acombination thereof.

Excess inner electrode material from the coating step can be removed toprovide a uniform thickness of the inner electrode 4. The innerelectrode then serves as a substrate onto which the green un-sinteredceramic layer 5 is deposited or coated.

The green ceramic layer or shell 5 is coated or deposited directly ontop of the inner electrode 4, preferably by contacting or dipping thesubstrate into a well-dispersed ceramic slurry.

The ceramic slurry contains a solvent, and a plurality of suitableceramic particles. Optionally, the slurry contains at least one suitableco-solvent, binder, dispersant, antifoam, or a combination thereof.

Suitable ceramic particles can include, but are not limited to, PZT,PMN, metal titanates including: barium titanate, bismuth titanate,bismuth iron titanate, and a combination thereof. The ceramic particleshave an average size less than about 50 microns, preferably less thanabout 10 microns.

The ceramic layer 5 has a thickness greater than about 0.001 mm. Thelayer thickness can be modified by the time the core is placed incontact with the slurry in a particular contacting step, also by thenumber of contacting steps conducted. The coating process typicallyinvolves a single immersion of the core in the slurry for approximatelyone second.

The loading of ceramic particles in the slurry can also control theceramic layer thickness. The solids loading in the ceramic slurry isfrom about 1% to about 90% by volume, preferably from about 20% to about55% by volume.

Suitable solvents and co-solvents in the slurry include, water, andorganic solvents including, but not limited to alcohols, ketones,esters, hydrocarbons, aromatic hydrocarbons, amines, hetero-aromatichydrocarbons, and mixtures thereof.

Suitable binders can include, but are not limited to, wax, polyethyleneglycol, paraffin, polyvinyl alcohol, methyl cellulose, starch, and acombination thereof. The total amount of binder in the slurry is lessthan about 10% by volume, preferably less than about 1%.

Suitable dispersants that stabilize the slurry, adjust pH, and preventformation of agglomeration can include, but are not limited to, cationicsurfactants, non-ionic surfactants, anionic surfactants, carbonic acidand salts, silicic acid and salts, polyacrylate and salts, citric acidand salts, polymethacrylate and salts, and a combination thereof.Dispersants are less than about 10% of the total slurry by volume,preferably less than about 2%.

Suitable antifoams or defoaming agents can include, but are not limitedto, fluorocarbons, dimethylsilicones, higher molecular weight alcohols,glycols, salts of stearic acid, and a combination thereof. Antifoam isless than about 5% by volume of the total slurry, preferably less thanabout 0.5%.

The viscosity of the ceramic slurry is less than about 5000 mPa·s,preferably less than about 2000 mPa·s.

After coating the inner electrode containing core 10 with the ceramicslurry, excess slurry can be removed by mechanical means, such asspinning, to insure formation of a uniform green ceramic layer.

It is important to note that a porous polystyrene core and the innerelectrode both absorb water from the wet ceramic layer. Capillary actionproduces a dense green ceramic layer, which is crucial to fabricating afully densified ceramic body.

After coating the core 1 with the inner electrode 4 and the ceramiclayer 5 to produce unit 20 as shown in FIG. 1 c, the layers can be driedvia the application of heat. Preferably, the unit 20 is subjected to thethermal core removal step, commonly referred to as core burnout, whereinthe core is thermally removed from within the layers by subjecting theunit 20 to temperatures from about 100° C. to about 900° C., preferablyfrom about 300° C. to about 700° C., for a period of time sufficient tothermally remove the core 1, solvents, co-solvents, binders, antifoams,and dispersants from within the unit 20 to produce a hollow unit 30 asshown in FIG. 1 d. This step can be carried out in a metal vapor richatmosphere to prevent metals contained in the ceramic layer 5 from beingleached out. Optionally, the thermal removal step can be carried out inan essentially oxygen free environment to prevent oxidation of firstconductor 2 and inner electrode 4. The step is carried out under suchconditions that the inner electrode 4 and the ceramic layer or shell 5remain in contact and physically intact.

The temperature of the thermal core removal (burnout) step is preferablyincreased from ambient to a desired final temperature at rates fromabout 0.1° C./min to about 20° C./min, preferably at about 1° C./min.The unit is maintained at the final temperature until such time as thecore 1, solvents, co-solvents, binders, antifoams, and dispersants aresufficiently removed. The final temperature hold time is typically aboutseveral hours.

At this stage in the process, a hollow unit 30 is produced whichincludes a hollow green ceramic shell 5 with a protruding firstconductor 2 attached to an inner electrode 4 as shown in FIG. 1 d.

Next, hollow unit 30 is sintered under conditions that vary according tothe ceramic composition. Typically, the ceramic and electrode materialsare sintered in the temperature range from about 900° C. to about 2000°C., preferably from about 1200° C. to about 1400° C. The sinteringtemperature and time need to be adjusted experimentally to achieve thedesired density of the particular ceramic material and layer thicknessused. For metal containing ceramic compositions, i.e., lead, bismuth,barium, the ceramic layer can be sintered in a metal rich atmosphere toreduce the metal loss from the ceramic body. Optionally, hollow unit 30can be sintered in an essentially oxygen free atmosphere to preventoxidation of the first conductor and the inner electrode. The uppertemperature limit of the sintering step should not exceed the meltingpoint of the inner electrode, else a uniform inner electrode in contactwith the ceramic layer cannot be obtained.

After sintering and cooling, the outer surface of the sintered ceramicshell 15 (see FIG. 2) is coated with an outer electrode 6. The outerelectrode is preferably metal and most preferably silver metal. Coatingcan be accomplished by contacting the sintered surface with the metal,or a metal ink through sputtering or dipping. A second conductor 7,preferably a thin lead wire can then be electrically attached to outerelectrode 6.

The sintered ceramic shell 15 is then poled with an electric fieldhaving a strength from about 500 V/mm to about 5000 V/mm, preferablyfrom about 1000 V/mm to about 3000 V/mm. The poling can be carried outat elevated temperatures in the range from about 50° C. to about 300° C.This completes the process of fabricating a hollow ceramic electroactivetransducer 40.

The cores recited in the present invention can be of any suitablegeometric shape, for example, spherical cores for hollow spheres,cylinder cores for tubes, cubic cores for hollow cubes etc.

In a further embodiment of the present invention, the ceramic coresmentioned above can be machined or ground to produce derivative shapesat any point during the process. These derivative shapes being definedby the original shape of the core as intersected by at least one planepartially, or completely therethrough. For the purpose of example only,a hollow cylinder shaped green ceramic or sintered ceramic can have theends removed, either prior to or after poling in the case of thesintered ceramic, to produce a hollow tube shaped ceramic electroactivetransducer. A spherical device can also be ground or machined to producea hemi-spherical or shell-shaped ceramic electroactive transducer.

The present invention can also produced a multilayered hollow ceramicelectroactive transducer as shown in FIG. 5. In addition to a firstconductor 2, an inner electrode 4, and a ceramic layer 15, additionalconductors 7, and alternating layers of electrodes 9, and ceramic layers16 are fabricated by repeating the steps of attaching a conductor 7 togreen ceramic layer 5 with subsequent coating of ceramic layer 5 withadditional electrode 9, followed by coating additional electrode 9 withadditional ceramic layer prior to the core removal step with subsequentsintering as described above. Upon sintering, the most outer ceramiclayer 16 is coated with outer electrode 6, and the sintered ceramiclayers 15 and 16 are poled as described above to produced themulti-layered ceramic electroactive transducer 50.

Referring to FIG. 6 a, a plurality of cores can be attached to at leastone conductor 25 or other support 26 to fabricate a predeterminedspatial relationship, i.e., two-dimensional or three-dimensional array,of ceramic electroactive transducers. As shown in FIG. 6 b, the innerelectrodes can be in electrical communication with one conductor,optionally the inner electrodes can be electrically isolated withone-another through the use of a plurality of conductors 25 a, 25 b and25 c as shown in FIG. 7.

The size limit of the present invention is controlled by the size of thecore, and the ability to coat the cores in a uniform manner. Hollowceramic electroactive transducers having diameters of several tenths ofa millimeter are easily fabricated using the steps of the presentinvention.

The present invention is further described in the following examples,which are intended to be illustrative and not limiting.

EXAMPLE 1

Deionized water was added to PZT powder containing PVA binders(available from Vernitron Piezoelectric Division) to form a ceramicslurry with a solids loading of 20 vol %. The slurry was ball milled tobreak up agglomerates and aggregates followed by sieving and evacuationto remove entrapped air. A porous polystyrene sphere of 2.5 mm indiameter was secured to the end of a 0.4 mm diameter platinum wire usingPVA as the binder. The inner electrode was coated onto the core bydipping the core into commercially available platinum ink (availablefrom Ferro Electronic Materials Division, El 192 Pt Internal Electrode)that had been diluted with acetone. The inner electrode coated spherewas then dried at room temperature prior to being dipped into the aboveceramic slurry. The excess slurry was removed by spinning the dippedcore at the end of the Pt wire.

After being dried at room temperature, the core was thermally removed byplacing the unit in an oven and increasing the temperature from ambientto 350° C. at 1° C./min. The unit was held at 350° C. for one hour, andthen the temperature was increased to 550° C. at 1° C./min. The unit wasthen held at 550° C. for an additional two hours. A hollow green-ceramiclayer with an inner electrode was obtained. The hollow unit was thensintered at 1285° C. for 1.5 hours in a closed alumina crucible with aPZT powder bed.

After cooling the sintered unit, the platinum wire was coated with aninsulating epoxy prior to coating the outer layer on with an air-drysilver ink. A thin silver lead wire was attached to the outer electrodewith silver conductive epoxy.

The sintered ceramic layer was then poled under an electric-fieldstrength of 2000 V/mm, while being heated to 150° C. in a silicon oilbath. The hollow ceramic electroactive transducer was then encapsulatedin a non-conductive resin and evaluated.

As shown in FIG. 3, the hollow sphere ceramic electroactive transducerof Example 1 had a measured capacitance of 1.4 nF, which is in agreementwith the calculated value. FIG. 4 shows the measured admittance of thishollow sphere ceramic electroactive transducer.

EXAMPLE 2

A spherical multilayer ceramic electroactive transducer as shown in FIG.5, was fabricated by following the process of Example 1, with theexception of attaching an additional conductor to the first ceramiclayer which was then coated with an additional electrode, followed bycoating with an additional layer of ceramic as described in Example 1above. Care was taken to insure the second electrode layer did not touchthe platinum wire used for the first conductor.

The core was then thermally removed and the hollow unit sintered as inExample 1 above. An outer electrode was coated on the outer ceramiclayer, and the sintered ceramic layers poled as outlined above.

It was unexpectedly discovered that connecting the inner electrode inparallel to the additional electrode enhanced the pressure output of thetransducer, and connecting the two electrodes in series enhanced thereceiver sensitivity of the transducer.

EXAMPLE 3

A linear array of hollow spherical ceramic electroactive transducers wasfabricated by first providing an array of 2.5 mm polystyrene sphericalcores threaded on a Pt wire having a 0.4 mm diameter. As described inExample 1, the cores were coated with an inner electrode and thencoating with a ceramic layer prior to thermally removing the core andsintering the array of connected hollow units. The sintered ceramiclayers of the array were then coated with an outer electrode and poledto produce an array of hollow ceramic electroactive transducers. Thearray was then encapsulated in a non-conductive resin to maintain thedesired spatial arrangement.

The present invention has been described with particular reference tothe preferred embodiments. It should be understood that variations andmodifications thereof can be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention.Accordingly, the present invention embraces all such alternatives,modifications and variations that fall within the scope of the appendedclaims.

1-36. (canceled)
 37. A process for fabricating a ceramic electroactivetransducer of a predetermined shape comprising the steps of: (a)providing a suitably shaped core having an outer surface; (b) attachinga first conductor to said outer surface of said core; (c) coating afirst inner conductive electrode on said outer surface of said core suchthat said first inner conductive electrode is in electricalcommunication with said first conductor; (d) coating a first ceramiclayer having an outer surface on said first inner electrode; thereafter(e) attaching a second conductor to said outer surface of said firstceramic layer; (f) coating a second inner conductive electrode on saidouter surface of said first ceramic layer such that said second innerconductive electrode is in electrical communication with said secondconductor without being in electrical communication with said firstconductor; (g) coating a second ceramic layer having an outer surface onsaid second inner electrode; thereafter (h) sintering said first andsaid second ceramic layers; (i) coating an outer electrode on saidsecond sintered ceramic layer to produce an outer electrode that is notin electrical communication with said first conductor or with saidsecond conductor; (i) poling said first and said second sintered ceramiclayers across said first inner electrode, said second inner electrode,and said outer electrode; and (k) adding a plurality of additionalconductors, electrodes, and ceramic layers prior to said sintering step.38. The process of claim 37, wherein said plurality of additionalelectrodes are electrically isolated from each other, from said innerelectrode, and from said outer electrode. 39-40. (canceled)
 41. Aprocess for fabricating a plurality of ceramic electroactive transducershaving a predetermined spatial relationship of a predetermined shapecomprising the steps of: (a) providing a plurality of suitably shapedcores, each having an outer surface, arranged in said predeterminedspatial relationship; (b) attaching one or more first conductors to saidcores; (c) coating an inner conductive electrode on each of said outersurfaces of said cores such that said inner conductive electrodes are inelectrical communication with at least one of said first conductors; (d)coating a ceramic layer on each of said inner electrodes; thereafter (e)sintering said ceramic layers; (f) coating one or more outer electrodeson each of said sintered ceramic layers to produce a plurality of outerelectrodes not in electrical communication with said first conductor;and (g) poling said sintered ceramic layers across said inner electrodesand said plurality of said outer electrodes.
 42. The process of claim41, wherein said predetermined spatial relationship is changeable at anystep of said fabricating process.